Hermetic feedthrough for an implantable medical device

ABSTRACT

A hermetic feedthrough for an implantable medical device includes a sheet having a hole defined therethrough, wherein the sheet comprises a first material that is an electrically insulative ceramic comprising alumina. The feedthrough further includes a second material substantially filling the hole so as to form a conduit, the second material having platinum and an additive that includes alumina. The second material does not include SiO 2 , MgO, or CaO. The first and second materials have a co-fired bond therebetween, the co-fired bond hermetically sealing the hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/203,241, filed Mar. 10, 2014, which is a Continuation of U.S. patentapplication Ser. No. 13/196,695, filed Aug. 2, 2011 (now U.S. Pat. No.8,670,829), both of which are incorporated by reference herein in theirentireties.

BACKGROUND

Technology disclosed herein relates generally to the field offeedthroughs serving as an electrical interface to connect portions of acircuit on opposite sides of a barrier. More specifically, technologydisclosed herein relates to hermetic feedthroughs for use withimplantable medical devices that are constructed through a co-firingprocess with a combination of materials selected to be bothbiocompatible and biostable over a long duration.

SUMMARY

One embodiment relates to a method of manufacturing an implantablemedical device, which includes manufacturing a hermetic feedthrough byproviding a first sheet of unfired ceramic material, forming one or moreholes through the sheet, inserting a first conductive material into oneof the holes, and forming a pad in electrical contact with the firstconductive material in one of the holes, wherein the pad comprises aplurality of layers and has a thickness of at least 50 micrometers, atleast one layer comprising a second conductive material having adifferent composition than the first conductive material. The methodfurther includes co-firing the unfired ceramic material, the firstconductive material, and the second conductive material. The methodfurther includes coupling the feedthrough to an encasement structure ofthe implantable medical device.

Another embodiment relates to a hermetic feedthrough for an implantablemedical device, which includes a sheet having a hole definedtherethrough, wherein the sheet comprises a first material that is anelectrically insulative ceramic comprising alumina. The feedthroughfurther includes a second material substantially filling the hole so asto form a conduit, the second material comprising platinum and anadditive that comprises alumina, and wherein the second material doesnot include SiO₂, MgO, or CaO. The first and second materials have aco-fired bond therebetween, the co-fired bond hermetically sealing thehole.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view of a medical device implanted within apatient according to an exemplary embodiment.

FIG. 2 is schematic view of another medical device implanted within apatient according to an exemplary embodiment.

FIG. 3 is a perspective view of a portion of a medical device includinga feedthrough according to an exemplary embodiment.

FIG. 4 is an exploded view of a portion of a medical device according toanother exemplary embodiment.

FIG. 5 is a perspective view of the portion of the medical device ofFIG. 4.

FIG. 6 is a sectional view of the portion of the medical device of FIG.4, taken along line 6-6 as shown in FIG. 5.

FIG. 7 is a sectional view of the portion of the medical device of FIG.4, taken along area 7 as shown in FIG. 6.

FIG. 8 is a perspective view of a portion of a medical device accordingto yet another exemplary embodiment.

FIG. 9 is a sectional view of the portion of the medical device of FIG.8, taken along line 9-9 as shown in FIG. 8.

FIG. 10 is a perspective view of a feedthrough according to an exemplaryembodiment.

FIG. 11 is a top view of the feedthrough of FIG. 10.

FIG. 12 is a bottom view of the feedthrough of FIG. 10.

FIG. 13 is a side view and partial sectional view of the feedthrough ofFIG. 4, where the sectional view is taken along line 13-13 in FIG. 11.

FIG. 14 is a sectional scanning electron microscopy (SEM) micrograph ofan interface between a conductive conduit and a insulator according toan exemplary embodiment.

FIG. 15 is a sectional SEM micrograph of an interface between aconductive conduit and a insulator according to another exemplaryembodiment.

FIG. 16 is a perspective view of a conductor of a feedthrough accordingto an exemplary embodiment.

FIG. 17 is a sectional SEM micrograph of a portion of a feedthroughaccording to an exemplary embodiment.

FIG. 18 is a sectional SEM micrograph of another portion of afeedthrough according to an exemplary embodiment.

FIG. 19 is a top view SEM micrograph of a pad of a feedthrough accordingto an exemplary embodiment.

FIG. 20 is a series of sectional SEM micrographs and correspondingdiagrams of portions of feedthroughs according to exemplary embodiments.

FIG. 21 is a perspective view of a process of manufacturing afeedthrough according to an exemplary embodiment.

DETAILED DESCRIPTION

Before turning to the figures, which illustrate the exemplaryembodiments in detail, it should be understood that the presentapplication is not limited to the details or methodology set forth inthe description or illustrated in the figures. It should also beunderstood that the terminology is for the purpose of description onlyand should not be regarded as limiting.

Referring to FIG. 1, an implantable medical device 110, such as apacemaker or a defibrillator, includes a base 112 (e.g., pulsegenerator, main body) and leads 114. The device 110 may be implanted ina human patient 116 or other being. In some embodiments, the device 110is configured to provide a therapeutic treatment in the form of anelectrical pulse, which in some embodiments may be on the order of about700 volts. In contemplated embodiments, the device 110, or a variationthereof, may be used to treat or monitor a wide range of conditions suchas pain, incontinence, hearing loss, movement disorders includingepilepsy and Parkinson's disease, sleep apnea, and a variety of otherphysiological, psychological, and emotional conditions and disorders.

Within the base 112, the device 110 may include components, such ascontrol circuitry and energy storage devices (e.g., secondary battery,capacitor), that may not be biocompatible or able to function when wet.However, according to an exemplary embodiment, the base 112 ishermetically-sealed and formed with an exterior of a biocompatible andbiostable material (e.g., titanium, biocompatible coating) isolating theinterior of the base 112 from bodily fluids of the patient 116 that areoutside the base 112. In some embodiments, the base 112 further includesa hermetic feedthrough 118 (e.g., through-connection, interface,connector, coupling) formed from or including an exterior of abiocompatible and biostable material. The feedthrough 118 facilitateselectric transmission through the base 112, from the interior of thebase 112 to the exterior of the base 112 and vice versa.

By way of example, during use of the implantable medical device 110, acharge stored in a capacitor interior to the base 112 may be dischargedin the form of an electrical pulse. The electrical pulse is transferredthrough a wall of the base 112 via the feedthrough 118. The electricalpulse is then received by at least one of the proximal ends 120 of theleads 114 and transmitted via conductive pathways through at least oneof the leads 114 to electrodes 122, which may be located at distal endsof the leads 114. The electrodes 122 may be coupled to a heart 124 orother part(s) of the patient 116 to promote a pattern of heartbeats,stimulate heartbeats, sense heartbeats, promote healing, or for otherreasons.

In some embodiments, activity is sensed via the electrodes 122 andcommunicated by the leads 114 to control circuitry in the base 112 viathe feedthrough 118. The sensed activity may be used as feedback by thecontrol circuitry to manage the operation of the device 110. In stillother embodiments, the feedthrough 118 may also be used to facilitatetransfer of electricity to the energy storage device within the base112, such as for recharging or testing. In other embodiments, otherenergy storage devices may be used, such as a hybrid system using acombination of a battery and a capacitor for energy storage. Accordingto an exemplary embodiment, two or more leads may be coupled to theinterior of the base 112 via the feedthrough 118. In other embodiments,a single lead may be used (see generally device 210 as shown in FIG. 2).

Referring to FIG. 2, an implantable medical device 210 (e.g., electricalstimulator, neurostimulator) is configured to influence a nervous systemand/or organs of a patient 212. The device 210 may be implanted, forexample, into a subcutaneous pocket in an abdomen 214, pectoral region,upper buttocks, or other area of the patient 212, and the device 210 maybe programmed to provide a stimulation signal (e.g., electrical pulse,frequency, voltage) associated with a specific therapy. During use,electrical contacts integrated with a lead 216 are placed at a desiredstimulation site, such as a portion of a spine 218 or brain. The lead216 is also connected to a base 220 of the device 210 by way of afeedthrough 222 integrated with an exterior surface of the base 220. Insome contemplated embodiments, a feedthrough can transmit therapy and/orsend signals directly to electrodes mounted on the implantable medicaldevice (e.g., so-called leadless devices).

According to an exemplary embodiment, the feedthrough 222, as well asthe rest the exterior of the base 220, is designed to be hermeticallysealed, biocompatible, and biostable in order to prevent leakage ofbodily fluids to the interior of the base 220, as well as to preventleakage from the interior of the base 220 into the body during theusable life of the implantable medical device 210. According to anexemplary embodiment, the feedthrough 222 is hermetically sealed, andremains hermetically sealed when implanted in the body, displayinglong-term biostability on the order of years, such as at least a year,five years, ten years, twenty years, or more.

Standard testing, such as in-vitro highly-accelerated immersion testingfor hermeticity and dye infiltration, may be used to provide a reliableindicator of the ability of the feedthroughs 118, 222 to remainhermetically sealed and biostable when implanted over an extendedperiod. Long-term hermeticity and/or biostability may be demonstrated bythe occurrence of substantially no dye infiltration and substantially noloss of the hermetic seal (i.e., evidenced by the absence of dyepenetration, helium leak, etc.) through the feedthrough after immersionin simulated body fluid at a controlled temperature (e.g., 120° C., 150°C., 200° C. or more) and pressure (e.g., 1.5 atm, 3.5 atm) over anextended test duration (e.g., 48 hours, 72 hours, 96 hours, a month ormore), while maintaining high electrical conductivity through thefeedthrough 222. Other standard tests, such as a Helium leak test and a3-point bending strength test, may also evidence long-term biostability,as may be indicated by minimal degradation of strength and retention oflow Helium leak rates, typically less than 1×10⁻⁸ atm-cc He per second(e.g., less than 5×10⁻⁹ atm-cc He per second).

Although described herein with respect to particular implantable medicaldevices, it should be understood that the concepts disclosed herein maybe utilized in conjunction with a wide range of implantable medicaldevices, such as pacemakers, implantable cardioverter-defibrillators,sensors, cardiac contractility modulators, cardioverters, drugadministering devices, diagnostic recorders, cochlear implants, andother devices. According to still other contemplated embodiments,devices other than implantable medical devices may also benefit from theconcepts disclosed herein.

Referring now to FIG. 3, a wall 310 or encasement structure of animplantable medical device (see, e.g., implantable medical devices 110and 210 as shown in FIGS. 1-2) includes a feedthrough 312. Thefeedthrough 312 is fastened to a portion 314 of the wall 310, such as aferrule, in a recess 316 of the wall 310 that is configured to receivethe feedthrough 312. The wall 310 may be integrated with another wall orwalls to form a biocompatible, hermetically-sealed exterior for a base(see, e.g., bases 112 and 220 as shown in FIGS. 1-2) of the implantablemedical device. In other embodiments, a ferrule does not include arecess. In still other embodiments, a feedthrough may be integrateddirectly into a wall, without use of a ferrule.

According to an exemplary embodiment, the feedthrough 312 is primarilyformed from a material 318 that is generally electricallynon-conductive, an insulator, or a dielectric. The feedthrough furtherincludes one or more conduits 320 (e.g., conductive member, verticalinterconnect access (via), path, pathway) that are generallyelectrically conductive and that extend through the material 318 of thefeedthrough 312 that is generally electrically non-conductive. In somecontemplated embodiments, the conduits 320 are integrated with thematerial 318 but do not extend through the material 318, and insteadextend along a surface of the material 318, or on the surface of anintermediary material between the conduits 320 and the surface of thematerial 318. In this manner, the electrical signal can be conducted ina horizontal direction between conductive conduits (e.g., vias) orexternal pads, or otherwise connecting internal and/or external pointsthat are laterally disposed from one another.

Referring to FIGS. 4-5, components of an implantable medical device 1110include a feedthrough 1112 (e.g., co-fired ceramic, monolith), a ring1114 of filler material for brazing, and a ferrule 1116. During assemblyof the implantable medical device 1110, the feedthrough 1112 is insertedinto a recess 1118 (e.g., opening) in the ferrule 1116, the ring 1114 isthen melted and brazed between the feedthrough 1112 and the ferrule1116. In some embodiments, the ring 1114 is a gold ring, and the ferrule1116 is formed from titanium. Gold and titanium are used in someembodiments due to the associated biocompatible properties and relativemelting temperatures. In some embodiments, side walls of the ceramicinsulator are coated (e.g., by a variety of potential methods, such asphysical vapor deposition, sputtering, electron-beam evaporation,plating, chemical vapor deposition) with a metal, such as niobium,titanium, molybdenum, or other biocompatible materials, to facilitatejoining between the insulator and the ferrule. The coat of metal mayfacilitate adhesion and brazing of a pre-form gold ring to join theinsulator and ferrule. In other contemplated embodiments, a ring andferrule are formed from different materials or combinations ofmaterials.

Referring to FIGS. 6-7, the feedthrough 1112 includes conductiveconduits 1120 (e.g., via) extending through an insulator 1122, betweentop and bottom surfaces of the feedthrough 1112. In some embodiments, atleast one of the conductive conduits 1120 extends partially through theinsulator 1122, and couples to a horizontal conduit 1124 (FIG. 7) thatextends laterally to a side of the feedthrough 1112. In otherembodiments, a conduit may extend fully through a feedthrough, such asfrom a top to a bottom and still connect horizontally to another body.In FIG. 7, the horizontal conduit 1124 extends to the ring 1114, brazedbetween the ferrule 1116 and feedthrough 1112. Accordingly, thehorizontal conduit 1124 may serve as a ground plane for the feedthrough1112. In some embodiments, the conductive conduits 1120, including thehorizontal conduit 1124, include platinum. In some such embodiments, thehorizontal conduit 1124 is printed onto a layer of un-fired (e.g.,green) ceramic material, and co-fired with the other conductive conduits1120 and insulator 1124.

Referring to FIGS. 8-9, a co-fired feedthrough 1212 includes asubstantially rectangular insulator body 1214 with conductive conduits1216. The feedthrough 1212 has been brazed into a ferrule 1218 of animplantable medical device 1210 with a ring 1220 of a biocompatiblematerial. The prismatic shape of the rectangular insulator body 1214(FIG. 8) is believed to improve the braze joint stability, as discussedin more detail below. According to an exemplary embodiment, the ferrule1218 is ledge-less, where the insulator body is not supported by aflange or extension on the underside of the ferrule 1218, as compared tothe ferrule 1116 having a ledge 1126 as shown in FIG. 7. The ledge-lessdesign of the ferrule 1218 is intended to improve electrical isolationof the conductive conduits 1216 of the feedthrough 1212, by increasingthe path length for shorting between the conductive conduits 1216 andthe ferrule 1218, which is further intended to improve externalinterconnect access (e.g., a lead coupled to the feedthrough 1212).

Referring now to FIGS. 10-13, a feedthrough 410 is shown according toanother exemplary embodiment and includes a body 412 (e.g., insulator)and at least one conduit 414 (e.g., conductive pathway, electricalconduit, via). As shown, the feedthrough 410 includes eleven conduits414, but according to other embodiments may include a greater or lessernumber of conduits, or different layout of conduits. According to anexemplary embodiment, the body 412 is formed from a material that is anelectrical insulator, and in some embodiments the body 412 includessubstantially flat faces 416 (e.g., sides, exterior surfaces). The faces416 are separated from one another by corners 418 or edges.

According to an exemplary embodiment, the feedthrough 410 furtherincludes the conduit(s) 414 configured to conduct electricity throughthe electrical insulator material of the body 412. The conduit 414 maybe substantially straight or tortuous (e.g., staggered, serpentine,zigzag). A tortuous path for the conduit 414 may improve the hermeticseal of the feedthrough 410 by better impeding fluid from seeping (e.g.,passing, ingress) between the conduit 414 and the body 412. However, atortuous path may increase electrical resistance, decreasing efficiencyof the feedthrough 410 relative to a conduit with a substantiallystraight path (e.g., overlaying a straight line). In some embodiments,resistance of the metallization is less than about 30 mΩ, such as lessthan about 10 mΩ. In other embodiments, the resistance of themetallization is less than about 100 mΩ. The resistance of themetallization may vary as a function of the diameter of the conduit 414,the thickness of the body 412, materials, and other properties. In somedesigns, resistance is increased as a conduit is staggered or madetortuous in order to bolster hermeticity, however it has been found thata tortuous path, and the associated resistance losses, may beunnecessary given the proper combination of materials, design, andco-firing processes.

In some embodiments, the faces 416 and corners 418 of the body 412together form a substantially prismatic or rectilinear exterior formfactor for the feedthrough 410 in which at least some faces 416 of thebody 412 (e.g., top 420 relative to end 422) are substantiallyorthogonal to one another or substantially parallel with one another. Insome such embodiments, all of the faces 416 of the body 412 are eithersubstantially orthogonal or substantially parallel to one another. Inother embodiments, none of the faces are substantially orthogonal to oneanother. In still other embodiments, at least some faces are not flat.

According to an exemplary embodiment, the feedthrough 410 is provided inthe form of a box-like structure with rectangular faces 416, such as ablock, a brick, or a cube. In some such embodiments, the body 412includes the top 420, a bottom 426, and sides (e.g., ends 422 andlengthwise sides 428) extending between the top 420 and bottom 426. Eachof the sides 422, 428 includes a flat surface. In some embodiments, theflat surfaces of the ends 422 are substantially the same size and shapeas one another, and the flat surfaces of the lengthwise sides aresubstantially the same size and shape as one another. In othercontemplated embodiments, a feedthrough is generally cylindrical, oval,or otherwise shaped.

Still referring to FIGS. 10-13, the flat surfaces of the ends 422 of thebody 412 are parallel to one another. In some such embodiments, the top420 and bottom 426 of the body 412 include flat surfaces orthogonal tothe flat surfaces of the ends 422 of the body 412. In some suchembodiments, the sides 428 extending lengthwise along the body 412include flat surfaces that are also orthogonal to the flat surfaces ofthe ends 422 of the body 412. According to such an embodiment, threecross-sections of the body 412 that are orthogonal to one another, eachhave substantially rectangular peripheries. For example, one rectangularcross-section extends in a lengthwise direction, another extends acrossthe width of the body, and a third rectangular cross-section cuts thebody along a horizontal plane.

According to an exemplary embodiment, the body 412 of the feedthrough410 further includes the corners 418 between the faces 416 of theexterior of the feedthrough 410. The corners 418 and edges may beright-angle corners, or may be otherwise angled. In some embodiments,the corners 418 are rounded (e.g., radiused, smoothed, dulled).According to an exemplary embodiment, the corners 418 are rounded bytumbling, grinding, milling, polishing, or another shaping process afterthe body 412 is cut into a rectilinear shape. In such embodiments, thecorners 418 are gradually worn by an abrasive agent, such as siliconcarbide grit. Controlled and limited application of the shaping processmay sufficiently maintain a relatively precise geometry of the body 412,while reducing the potential for stress concentrations and crackinitiation sites provided by the corners 418. Controlled and limitedshaping may also reduce the potential for damage to occur below thesurface of the insulator body 412. However, in still other contemplatedembodiments, the corners may be beveled or sharp.

According to an exemplary embodiment, the body 412 of the feedthrough410 is formed from a ceramic material, and the conduit 414 is formedfrom a metallic paste (e.g., via paste). During manufacturing of thefeedthrough 410, the metallic paste of the conduit 414 is filled into ahole 424 (e.g., square hole, round hole, oval hole, etc.) in the ceramicmaterial of the body 412 (see generally FIG. 21 discussed below). Thebody 412 and conduit 414 of the feedthrough 410 are then co-fired—boththe ceramic material of the body 412 and the metallic paste of theconduit 414 are fired together in a kiln at the same time, such as at atemperature of about 1600° C. for about an hour.

According to an exemplary embodiment, the material of the body 412includes alumina (e.g., aluminum oxide, corundum), such as at least 70%alumina or about 92% or 96% alumina. In some embodiments, the metallicpaste of the conduit 414 primarily includes platinum (e.g., platinumpowder) and an additive, where the additive comprises alumina (e.g., d₅₀of 1-10 μm alumina powder). The metallic paste may include a firstplatinum powder having a median particle size between 3 to 10 μm (e.g.,d₅₀ median particle size), a second, coarser platinum powder having amedian particle size between 5 to 20 μm, or a combination of platinumpowders. In other contemplated embodiments, such as those that may ormay not be intended for use in an implant, the paste may includetitanium, niobium, zirconium, tantalum, other refractory metals, alloysthereof, oxides thereof, or other materials.

Use of different size particles for the materials of the metallic paste,including additives, is believed to change the thermal expansionresponse and/or sintering kinetics and properties (e.g., sinteringshrinkage, shrinking profile) of the metallic paste, which may beadjusted as necessary to be compatible with the other materials of theco-fired feedthrough, such as the material of the body 412. Furthermorein some embodiments, during the co-firing process, alumina of the body412 is sintered, and the alumina that is an additive of the metallicpaste may improve adhesion between the metallic paste of the conduit 414and the alumina of the body 412 forming a strong co-fired bondtherebetween.

On a micro-scale, the alumina in the metallic paste may bond with thealumina of the body 412 along the border (e.g., boundary, interface)between the conduit 414 and the body 412 in the hole 424 (see generallyscanning electron microscopy as shown in FIGS. 14-15). The bond formedwith the alumina as an additive in the metallic paste is believed tosignificantly improve the hermetic seal, when compared to the bondbetween the conduit 414 and the body 412 without alumina as an additivein the metallic paste, because of the interaction between the aluminaadditive and alumina of the body 412. Inclusion of alumina as anadditive in the metallic paste may reduce the size and quantity ofvoids, and may also improve the thermal expansion compatibility of themetallic paste with the ceramic of the body 412 during the co-firingprocess, reducing stresses otherwise caused by unequal expansions orcontraction of the different materials of the feedthrough 410 andforming a hermetic, biostable co-fired bond.

Due at least in part to the combination of materials selected for thebody 412 and conduit 414, the result of the co-firing process is thatthe conduit 414 is hermetically sealed with the body 412. Fluids, suchas bodily liquids and gases, are prevented from passing through theconduit 414 or between the conduit 414 and the body 412 of thefeedthrough 410, such as through a chain of micro-pores at theinterface. Furthermore, the feedthrough 410 remains biostable, with thehermetic seal not breaking down over a long duration, on the order ofyears.

Referring to FIGS. 14-15, co-fired feedthroughs 1310, 1410 includeinterfaces 1312, 1412 between materials of conductive conduits 1314,1414 and insulator bodies 1316, 1416 that differ from one another atleast in part due to additives used in the materials of the conductiveconduits 1314, 1414. While the material of the insulator bodies 1316,1416 shown in FIGS. 14-15 is substantially the same (e.g., about 92%alumina), the materials of the conductive conduit 1314, shown in FIG.14, includes platinum (e.g., platinum powder in the form of a paste forco-firing) with additives of Al₂O₃, SiO₂, CaO, MgO, while the materialof the conductive conduit 1414, shown in FIG. 15, includes platinum withonly Al₂O₃ as an additive. The co-fired interfaces 1312, 1412 of FIGS.14-15 both include some initial defects 1318, 1418 (e.g., voids, pores).However, it has been found that use of only Al₂O₃ as an additivedecreases the quantity and/or magnitude of the initial defects 1418,providing an improved interface 1412 (e.g., co-fired bond).

For purposes of context and as summarized in TABLE 1, a set of viametallization compositions was evaluated for parameters relevant toco-firing and feedthrough performance, such as via projections,adhesion, warpage, resistivity, and hermeticity. Different additives andcombinations of additives (e.g., Al₂O₃ alone; Al₂O₃, SiO₂, MgO, and CaO;and SiO₂, MgO, and CaO) were provided to a paste of platinum atdifferent levels of concentration, ranging from 0-10% of the paste.

TABLE 1 Level Related to Additive Concentration (between 0-10%)Designation Additives Level 0 Level 1 Level 2 Level 3 A Al₂O₃ alone A1A2 A3 B Al₂O₃, SiO₂, P B1 B2 B3 MgO, and CaO C SiO₂, MgO, C1 C2 C3 andCaO

In the evaluation corresponding to TABLE 1, provided for context,hermeticity was evaluated using a He leak test before and after thermalshock testing, which included 5 and 500 cycles ranging from −50° C. to165° C. As shown in TABLE 2, the formulation corresponding to B2remained hermetically sealed according to the He leak test, even after500 cycles.

TABLE 2 Initial after after Composition Insulator He Leak 5 cycles 500cycles Pt + Alumina 20 of 20 20 of 20 20 of 20 5% (Al₂O₃, SiO₂, no leakno leak no leak MgO, and CaO)

For purposes of further context, a series of feedthrough parts(designated TS4.6, TS4.8 and TS 5) were manufactured using theformulation of TABLE 2. As summarized in TABLE 3, it was noted that,while the conductive conduits (e.g., vias) were initially hermetic,having passed the He leak tests, a significant fraction of conductiveconduits (e.g., up to 3%) was found to exhibit dye penetration down somelength of the via.

TABLE 3 Dye Diameter Length Pitch He Penetration (mil) (mil) (mm) PathLeak Pieces Via (via) TS4.6 6 42 0.51 straight pass 10 200 4 2.0% 6 420.79 straight pass 10 130 4 3.1% 6 42 0.46 straight pass 10 200 1 0.5% 642 0.56 straight pass 10 260 3 1.2% TS4.6 6 66 0.51 straight pass 10 2005 2.5% 6 66 0.79 straight pass 10 130 2 1.5% 6 66 0.51 straight pass 10200 5 2.5% 6 66 0.56 straight pass 10 260 4 1.5% TS4.8 8 66 0.64straight pass 10 170 5 2.9% 8 66 1.02 straight pass 10 110 3 2.7% 8 660.51 straight pass 10 200 3 1.5% 8 66 0.64 straight pass 10 200 6 3.0% 866 0.81 staggered pass 10 320 1 0.3% TS5 8 42 min. 0.71 straight pass 10280 0 0

In TABLE 3, the “diameter,” “length,” and “pitch” columns correspond tocharacteristics of the conductive conduits (i.e., via); the “path”column indicates whether the conductive conduits were stacked to form astraight or staggered path; the “He Leak” column indicates whether theconfiguration passed a He leak test; the “Pieces” and “Via” columnprovide the number of via tested as well as the number of correspondingpieces in which the via were located; and the “Dye Penetration” columnindicates the number of via exhibiting dye penetration and thepercentage of total via tested. Following the successful performance inthermal shock tests, evidence of dye penetration was unexpected. Toaddress this unexpected result of dye penetration shown in TABLE 3, anumber of factors potentially affecting via hermeticity were evaluated,including design and process conditions, in addition to inorganic andorganic components in the via paste. Of these, the inorganic additiveswere found to strongly influence the hermeticity of the resultingco-fired structure.

As summarized in the following table, provided for purposes of example,the percentage of alumina additive to paste of platinum powder having aparticle size distribution d₅₀ in the range of 3-10 μm (“Pt-1”)influenced the resistance of the conductive conduit (e.g., metallizationresistance of the via):

TABLE 4 Alumina 2.5%  3% 3.5%  4% 4.5%  5% Other 2.5% 0 0 0 0 0 Via <10μm >20 μm >20 μm >20 μm >20 μm >20 μm Projection Warpage <5 μm >50μm >50 μm >50 μm >50 μm >50 μm Shrinkage  15% 10%  10% 10%  10% 10%Penetrated 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 He Leak0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 Metallization 28.5× 10⁻⁸ Ω · m 15.1 × 10⁻⁸ Ω · m 16.4 × 10⁻⁸ Ω · m 17.7 × 10⁻⁸ Ω · m 19.0× 10⁻⁸ Ω · m 20.3 × 10⁻⁸ Ω · m Resistivity (line) Via 7.8 mΩ 6.8 mΩ 6.9mΩ 6.7 mΩ 6.2 mΩ 6.6 mΩ Resistance Adhesion alumina alumina aluminaalumina alumina alumina Failure Locationwhere the “Alumina” row includes the percentage of metallization pastethat is alumina; the “Other” row includes the percentage of themetallization paste that is alumina as well as other additives SiO₂,MgO, and CaO; the “Projection” row includes the height of the projectionof the via metallization; the “Warpage” row includes the magnitudes ofthe relative warpage of the substrate (e.g., insulator); the “Shrinkage”row includes the shrinkage of the metallization determined bythermo-mechanical analysis; the “Penetrated” row includes the number ofsamples that exhibited dye penetration; the “He Leak” row includes thenumber of samples that exhibited Helium leakage during testing; the“Metallization Resistivity” row includes the bulk electrical resistivityof the metallization; the “Via Resistance” row includes the electricalresistance of the conductive conduit, with other influencing factors,such as thickness (e.g., 66 mill) and diameter of the conduit heldsubstantially constant; and the “Adhesion Failure Location” row detailsthe location of failure in a standard soldered pin-pull test. While noneof the 700 the samples with 2.5% alumina additive showed dyepenetration, the samples with 5% alumina additive exhibited betterperformance. As shown in the examples of TABLE 4, it has been generallyfound that with the inclusion of alumina as an additive, and in theabsence of “Other” additives, such as SiO₂, MgO, and CaO, the adhesionimproved and electrical resistance decreased, but at a cost of increasedheight of via projections and increased sample warpage.

To mitigate the projection and warpage of the co-fired conductiveconduit (i.e., via), the use of different particle sizes of the platinumpowder for the metallization paste used to construct the conductiveconduits were screened. In some exemplary formulations, a coarserplatinum powder, having an average particle size distribution d₅₀ in therange of 5-20 μm (“Pt-2”), and/or mixed the Pt-2 powder with the Pt-1powder, was used, as summarized by the following table provided forcontext.

TABLE 5 Pt-1:Pt-2 1:0 0:1 1:0 1:0 9:1 4:1 Alumina 2.5% 0  4%  5%  4%  4%Other 2.5% 0 0 0 0 0 Via <10 μm >20 μm >20 μm >20 μm >20 μm ProjectionWarpage <5 μm 6 μm >50 μm >50 μm 43 μm 37 μm Shrinkage  15% 10% 10% 10%10% 11% Penetrated 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 He Leak0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 Metallization 28.5 × 10⁻⁸ Ω· m 20.7 × 10⁻⁸ Ω · m 17.7 × 10⁻⁸ Ω · m 20.3 × 10⁻⁸ Ω · m 18.7 × 10⁻⁸ Ω· m 19.3 × 10⁻⁸ Ω · m Resistivity (line) Via 7.8 mΩ 6.7 mΩ 6.6 mΩ 6.6 mΩ6.8 mΩ Resistance Adhesion alumina alumina alumina alumina aluminaalumina Failure Location

In TABLE 5, the “Pt-1:Pt-2” row includes the ratio of the two differentsize platinum powders used in the metallization paste, and the otherrows match those of TABLE 4. Mixing of the two platinum powders inratios of 9:1 and 4:1 decreased the relative projection and warpage ofthe co-fired conductive conduit from the insulator, but further decreasewas preferred in some embodiments. The mixtures of Pt-1 and Pt-2 incombination with alumina additive were refined as summarized thefollowing table provided for context:

TABLE 6 Pt-1:Pt-2 7:3 7:3 7:3 1:1 1:1 1:1 Alumina  5%  7%  9%  5%  7% 9% Other 0 0 0 0 0 0 Via 15 μm 15 μm 20 μm 10 μm 10 μm 15 μm ProjectionWarpage 10 μm 10 μm 10 μm <5 μm <5 μm <5 μm Shrinkage 13% 12% 11% 15%13% 12% Penetrated 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700He Leak 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700 0 of 700Metallization 20 × 10⁻⁸ Ω · m 23 × 10⁻⁸ Ω · m 27 × 10⁻⁸ Ω · m 21 × 10⁻⁸Ω · m 24 × 10⁻⁸ Ω · m 29 × 10⁻⁸ Ω · m Resistivity (line) Via 7.7 mΩ 8.1mΩ 10.2 mΩ 7.8 mΩ 9.2 mΩ 11.5 mΩ Resistance Adhesion alumina aluminaalumina alumina alumina alumina Failure LocationThe mixing of different size platinum powders and alumina additivesresulted in formulations for via paste with decreased projections,matched shrinkage (reduced warpage) as well as controlled resistance ofthe metallization.

For purposes of context, a formulation of platinum paste comprisingequal parts Pt-1 and Pt-2 platinum powders with 5% alumina additive wasused in the production of a number of substantially rectangular top andbottom pad constructions for feedthroughs, as summarized in thefollowing TABLE 7:

TABLE 7 Top Pad Top Pad Bottom Pitch Size Pad Size Dye (mm) (mm) (mm)Thickness Pene- x y x y x y (mm) tration 1 14.008 40.611 0.450 0.9530.784 0.784 1.660 0 of 108 2 14.015 40.610 0.445 0.948 0.785 0.790 1.6560 of 108 3 14.016 40.603 0.444 0.944 0.781 0.793 1.659 0 of 108 4 14.01340.615 0.453 0.942 0.788 0.792 1.660 0 of 108 5 14.028 40.647 0.4560.950 0.784 0.787 1.659 0 of 108 6 14.010 40.603 0.455 0.948 0.789 0.7931.661 0 of 108 7 14.018 40.596 0.455 0.950 0.790 0.794 1.661 0 of 108 814.007 40.628 0.447 0.952 0.786 0.789 1.666 0 of 108

The following TABLE 8 shows the results from biostability testing at150° C. in de-ionized water for 5 days, and subsequent thermal shocktests:

TABLE 8 Type of Test Quantity Tested Test Parameters Dye PenetrationImmersion 15 pieces 150° C. in de-ionized 0 of 15 pieces Testing (105via) water for 5 days (0 of 105 via) Thermal 15 pieces −65° C. to 150°C. 0 of 15 pieces Shock (105 via) for 1000 cycles (0 of 105 via)The samples remained hermetic without any dye penetration.

Referring back to FIGS. 10-13, the body 412 is formed from a material,such as alumina, that may be difficult to cut or shape due to thehardness of the material when fired. With such embodiments, arectilinear shape for the faces 416 the body 412 may be less difficultto form than a rounded shape. But, until recent discovery, a rectilinearshape was thought to promote structural weaknesses in a feedthrough andadjoining surfaces, such as due to increased stress concentrations andsusceptibility to cracking caused by sharp corners of the feedthrough.Accordingly, prior hermetic feedthroughs included rounded ends (e.g.,radiused ends, ovalized ends), which were time-consuming to form viagrinding processes or post-firing machining.

However, a feedthrough 410 having ends 422 of the body 412 with flatsurfaces has been discovered to improve the performance of the hermeticseal of the feedthrough 410 when integrated with or within a wall of animplantable medical device (see, e.g., FIG. 3), such as when metalizedand brazed into a ferrule incorporated into an implantable medicaldevice, without significant occurrence of the previously-feareddrawbacks of a rectilinear shape. It was surprising to find duringin-vitro, accelerated aging testing that durability significantlyincreased for the prismatic parts (e.g., flat ends), when compared toparts with radiused ends. Use of a body 412 with flat ends 422 isthought to improve the hermetic seal when the body 412 is integratedwith the ferrule. The flatness of the ends 422 is believed due at leastin part to the precision of the wafering saw with diamond cutting blades(or like instrument) used to cut of the material of the body 412, whencompared to a less-precise grinding process used to round ends of priorfeedthroughs. Additionally, the formation of flat, precisely cut ends422 is thought to reduce the likelihood of defects, pores, or voids onthe exterior surfaces of the body 412, which may provide a leak path forfluids.

By way of example, 50 rectangular bricks of insulator material were cutusing a wafering saw with target dimensions of 6.426 mm in length and1.778 mm in width. The average length of the 50 bricks was 6.437 mm witha standard deviation of 0.004 mm and the average width was 1.794 mm witha standard deviation of 0.004 mm. By contrast, in another set of 120samples with rounded ends formed from cutting with a wafering sawfollowed by grinding, the average length of 6.455 mm, which varied by astandard deviation of 0.011 mm. Subsequently, after polishing 100 of thesamples, the samples had an average length of 6.449 mm, which varied bya standard deviation of 0.010 mm. In width, the 120 samples with roundedends had an average width of 1.808 mm with a standard deviation of 0.007mm after grinding. Then after polishing, the 100 samples had an averagewidth of 1.795 mm with a standard deviation of 0.009 mm. As such, theuse of flat ends improved the dimensional accuracy of the insulator,while removing the additional manufacturing steps of grinding andpolishing.

The relative immersion performance of the cofired brick shape (flatsides with flat ends; see, e.g., FIGS. 8 and 10-13) to the cofiredradiused shape (flat sides with rounded ends; see, e.g., FIGS. 4-5) wasassessed via testing by immersing gold-brazed insulators (e.g.,gold-brazed ceramic, mostly alumina) into phosphate buffered saline(PBS) solution at 150° C. for up to 5 days. The same ferrule type/shape,gold preform, and brazing profile were used to braze the insulators ingold. Following the immersion period, the insulators were vacuum bakedand the helium leak rate for each insulators was measured. The pushoutstrength of the gold braze joint was also measured. Results of thetesting found that the radiused insulator shape lost up to 55% of itsoriginal pushout strength within 1.5 days at 150° C. in PBS solution,and about one-third of the radiused insulators leaked at rates fasterthan 5.0×10⁻⁹ atm*cc He/sec. In comparison, the brick insulator shapeshowed no reduction in pushout strength after 5 days at 150° C. in PBSsolution, with all parts remaining hermetic to better than 1.0×10⁻¹⁰atm*ccHe/sec. Accordingly, the testing indicated that the brick-shapedinsulators had superior immersion performance compared to the radiusedinsulators.

Referring to FIG. 21, a method 1010 of manufacturing a feedthrough 924includes providing sheets 1012 of green or un-fired ceramic material,forming 1014 holes in the ceramic sheets, and filling 1016 the holeswith a metallization or paste. In some embodiments, the method 1010includes printing covers or pads over the holes and metallization. Themethod 1010 further includes stacking 1020 and laminating 1022 thesheets, and then co-firing 1024 the ceramic and metallization. Themethod 1010 still further includes providing finished feedthroughs 1030by cutting 1026 the co-fired composition and rounding 1028 corners ofthe cut elements. The finished feedthroughs may then be brazed into aferrule and used as a portion of an implantable medical device.

In some embodiments, the method 1010 includes co-firing 1024 acomposition 914 (e.g., high-temperature co-fired ceramic, fired above1000° C., such as about 1600° C.; low-temperature co-fired ceramic,fired below 1000° C.) that includes a material 916 that is an electricalinsulator and a conduit(s) 918 configured to convey electricity throughthe electrical insulator material 916. The method 1010 further includescutting 920 (e.g., dicing, wafering) the composition 914 to form a body922 of a feedthrough 924. The insulator body 922 may then be processed1028 to form rounded corners 934 bordering a flat end surface 932.

In some embodiments, the body 922 has a top 926, a bottom (opposite tothe top 926), two sides 928 extending lengthwise along the body 922, andtwo sides 930 on ends of the body 922 (see also faces 416 of feedthrough410 as shown in FIGS. 10-13). In some such embodiments, the two sides930 on the ends of the body 922 include exterior flat surfaces 932. Themethod further includes rounding corners 934 between the two sides 928extending lengthwise along the body 922 and the two sides 930 on theends of the body 922. The corners 934 are rounded, but the two sides 930on the ends of the body 922 maintain the flat surfaces 932 between thecorners 934, providing 1030 the finished feedthrough.

In some embodiments, the method further includes filling holes 936 insheets 938 of the electrical insulator material 916, where the holes 936are filled with a conductive paste 940 used to form the conduit 918.Together, the sheets 938 and paste 940 are co-fired 1024 to form thefeedthrough 924, typically after stacking and laminating of the sheets938. The method 1010 further includes stacking 1020 the sheets 938 suchthat the holes 936 within each of the sheets 938 are substantiallyaligned with one another, forming a vertical path (see, e.g., conductiveconduit 614 as shown in FIG. 17). According to an exemplary embodiment,the method 1010 includes printing 944 a pad 946 overlaying the conduit918. In some embodiments, the pad 946 may serve as an interconnect ortop pad for the feedthrough 924, and may be formed from a series oflayers printed over one another to increase the thickness of the pad 946to a magnitude sufficiently thick to facilitate welding of a lead orwire to the pad 946 while maintaining the hermetic seal between the pad946 and the body 922 of the feedthrough 924 (see also FIG. 19). In otherembodiments, the pad 946 may serve as a cover pad to improveconnectivity between conduits 918 of adjacent sheets 938 (see, e.g.,cover pad 522 as shown in FIG. 16). In some embodiments, the sheets 938include alumina or are mostly formed from alumina, the conductive paste940 includes platinum and an additive, which may include alumina, and alayer of the pad 946 (e.g., cover pad and/or interconnect) is formedonly of platinum.

Referring now to FIG. 16, a feedthrough 510, which is configured to beused with an implantable medical device (see, e.g., devices 110, 210 asshown in FIGS. 1-2), includes a stack of sheets 512 (e.g., layers, ply,lamina, green sheets) that are laminated and fired together to form asingle solid body 514 (see also sectional view of feedthrough 410 asshown in FIG. 13). At least one of the sheets 516 is formed from a firstmaterial, and has at least one hole 518 extending through the sheet 516.According to an exemplary embodiment, the first material of the sheet516 is an electrical insulator material. A second material (e.g.,metallization) substantially fills the hole 518, such as filling atleast 75% of the volume of the hole 518 in contemplated embodiments. Thesecond material is conductive or is configured to be conductivefollowing firing, and forms an electrical conduit 520 through the sheet516. In some such embodiments, the first material is a ceramic, whichmay include alumina, and the second material is different than the firstmaterial and may include platinum and an additive.

According to an exemplary embodiment, the sheet 516 of the firstmaterial and the conduit 520 of the second material have been co-firedwith one another to at least partially form the feedthrough 510. Thecombination of first and second materials are selected to form a stronginterface (e.g., co-fired bond) with one another. According to anexemplary embodiment, chem-mechanical bonding between the first andsecond materials is sufficient for the second material of the conduit520 to hermetically seal the hole 518 in the sheet 516 of the firstmaterial as-fired, as-brazed, and after durability testing orimplantation in a human. In some embodiments, the additive of the secondmaterial includes the first material (e.g., ceramic, alumina), which isintended to promote chem-mechanical co-fired bonding between the firstand second materials during co-firing. In some such embodiments, thesecond material includes more platinum than alumina. In certainembodiments, the second material includes only platinum and alumina.

In at least some embodiments, the second material includes alumina, butdoes not include glass (or constituents thereof, such as SiO₂, MgO, CaO,crystalline oxides, or other constituents or glass) as an additive priorto co-firing. Typically glass is mixed with alumina to facilitatesintering of the alumina during firing. Typically glass is mixed withalumina to control sintering of the metallization during co-firing.However, it was discovered that glass is unnecessary to controlsintering of the metallization when alumina is used as an additive forthe second material. It is believed that the glass phase is drawn intothe second material (e.g., diffuses) from the surrounding first materialduring co-firing, which is believed to provide an intermingling ofmaterials along the interface, strengthening the chem-mechanicalco-fired bond at the interface between the first and second materials(e.g., via walls). Furthermore, it was discovered that use of glass asan additive may actually decrease the effectiveness of the hermetic sealbetween the first and second materials, because the glass is believed toproduce voids and other imperfections during firing of the secondmaterial, which may facilitate penetration of fluids through the secondmaterial or between the first and second materials of the feedthrough510. In other contemplated embodiments, the second material may includeglass.

Still referring to FIG. 16, the feedthrough 510 further includes a coverpad 522 (e.g., intra-layer pad, conductive disk, conduit extension)coupled to the sheet 516 and in electrical contact with the conduit 520.According to an exemplary embodiment, the cover pad 522 overlays thehole 518, and may extend at least partially over the sheet 516 past thehole 518. In other embodiments, cover pads are not included. In someembodiments, the cover pad 522 is formed from a third material (e.g.,metallization) that is different than both the first and secondmaterials. In other embodiments, the cover pad 522 is formed from thesecond material. The feedthrough 510 may also include external pads 536,538 formed from a stacked structure that may include the second materialand/or the third material. The third material is conductive and mayinclude platinum. In some embodiments, the third material includes onlyplatinum. In certain embodiments, the third material is more conductivethan the second material. In other contemplated embodiments, the thirdmaterial is the same as the second material.

According to an exemplary embodiment, the feedthrough 510 is formed froma combination of the sheets 512, which are stacked, laminated, and firedtogether. In some embodiments, the sheet 516 is a first sheet 516, andthe feedthrough 510 further includes a second sheet 524 and a thirdsheet 526, and possibly more sheets 512. As discussed, the first sheet516 is of the first material and has the hole 518, which is a first hole518. The second and third sheets 524, 526 are also formed from the firstmaterial. The second sheet is bonded to the first sheet 516, and thethird sheet 526 is fastened to the second sheet 524.

In such embodiments, the second sheet 524 has a second hole 528, and thethird sheet 526 has a third hole 530. As discussed, the first hole 518is filled with the second material, and according to an exemplaryembodiment, the second and third holes 528, 530 are also filled with thesecond material. Furthermore, the first, second, and third holes 518,528, 530 are vertically aligned with one another, in some embodiments,forming a substantially straight conductive path through the first,second, and third sheets 516, 524, 526. The first, second, and thirdholes 518, 528, 530 may substantially vertically overlap one another. Insome such embodiments, the first and second materials of the feedthrough510 have been co-fired such that a co-fired bond between the first andsecond materials hermetically seals the first, second, and third holes518, 528, 530, despite the conductive path being substantially straight.Accordingly, the conductive path has improved conductivity when comparedto tortuous paths of other feedthroughs, such as those of otherembodiments.

According to an exemplary embodiment, the cover pad 522 is a first coverpad 522, and the feedthrough 510 further includes a second cover pad 532and a third cover pad 534. The second and third cover pads 532, 534respectively overlay the second and third holes 528, 530 and at leastpartially extend over the second and third sheets 524, 526, past thesecond and third holes 528, 530. In some such embodiments, a staggeredconduit structure is contemplated, in which the first cover pad 522overlaps at least a portion of (e.g., is adjacent to, fully overlaps)the first and second holes 518, 528, which are not directly aligned withone another in a vertical stack, and the second cover pad 532 overlapsthe second and third holes 528, 530, which are also not directly alignedin a vertical stack. In other embodiments, the holes are directlyaligned with one another in a vertical stack. According to an exemplaryembodiment, the second and third cover pads 532, 534 are formed from thethird material. In other embodiments the second and third cover pads532, 534 are formed from the second material, or another material.

Referring now to FIG. 17, a section of an actual feedthrough 610,resembling the feedthrough 510, is shown in a scanning electronmicrograph. The feedthrough 610 includes a body 612 formed from aninsulator material, and a conductive conduit 614 extending through thebody 612. The conduit 614 has been formed by filling holes in the sheetswith a conductive material. The holes have been aligned with oneanother, and are capped by thin cover pads 616. According to anexemplary embodiment, the insulator material is a ceramic, includingalumina; the conductive material includes a mixture of platinum andalumina; and the material of the cover pads 616 also includes a mixtureof platinum and alumina. In other embodiments, the material of the pads616 primarily includes platinum. The body 612 has been formed from astack of sheets that have been laminated together and fired together ina kiln forming a co-fired bond therebetween. The materials have beenco-fired together to form a hermetic seal preventing fluids from passingthrough the feedthrough 610.

Referring again to FIG. 21, another portion of the method 1010 ofmanufacturing a feedthrough includes providing 1012 the sheet 938 offirst material 916, such as an electrical insulator material. In someembodiments, the sheet 938 is a ceramic that includes alumina. Themethod 1010 further includes forming 1014 (e.g., punching) at least onehole 936 in the first sheet 938, such as via a mechanical punch orpress. In some embodiments, an array of holes 936 are punched into thesheet 938, such as a second hole and a third hole in addition to thefirst hole.

According to an exemplary embodiment, the method 1010 includes filling1016 the hole 936 with the second material 940, which is different thanthe first material 916. In some embodiments, the second material 940 isconductive. In embodiments, with more than one hole 936, each of theholes 936 may be filled with the second material 940. When filling thehole 936, the second material 940 may be in the form of a paste, and mayinclude platinum and an additive, such as alumina. The method 1010includes co-firing 1024 the first and second materials 916, 940 suchthat a bond between the first and second materials 916, 940 hermeticallyseals the hole 936.

In some embodiments, the method 1010 may include providing additionalsheets 938 of the first material 916 (e.g., second and third sheets),forming holes 936 in each of the additional sheets 938, and stacking1020 the sheets 938. In some such embodiments, the sheets 938 arestacked such that corresponding holes 936 in the sheets 938 arevertically aligned with one another, forming a substantially straightconductive path through the first, second, and third sheets 938. Thesheets 938 are then laminated 1022 to one another and co-fired 1024 suchthat the first and second materials 916, 940 form a solid composition956 that is then cut or diced 1026 into individual bodies 922 that arehermetically sealed to prevent fluids from passing through the holes 936or between the first and second materials 916, 940.

In some embodiments, the method 1010 includes printing 1018 pads 946(e.g., cover pads) over the holes 936 and on the sheet 938, the pads 946extend at least partially past the hole 936. In some such embodiments,the pads 946 may be formed from a third material that is different fromthe first and second materials 916, 936. In some embodiments, the thirdmaterial includes platinum. In other such embodiments, the pads 946 maybe formed from the second material 940. If multiple sheets 938 are used,and corresponding holes 936 between sheets 938 are vertically aligned,then the pads 946 may serve to improve electrical connectivity betweenthe electrical conduits 918 of adjacent holes 936, especially if theholes 936 are not perfectly aligned with one another because of thelarger diameter of the pads 946. In some such embodiments, the first,second, and third materials are co-fired together during the co-firingstep 1024. In some embodiments, external or top pads may be printed overthe conduits 918 or base pads of the laminated structure 914. Theexternal pads may include a third material (e.g., metallization) that isdifferent than both the first and second materials. The third materialis conductive and may include platinum.

Referring now to FIGS. 18-19, a feedthrough 710 includes a body 712(e.g., electrical insulator), a conduit 714 (e.g., via) extendingthrough the body 712, and a pad 716 (e.g., top pad, base pad,interconnect) mounted to an exterior of the body 712, such as on a topor bottom surface of the body 712 and atop a base layer 718. Accordingto an exemplary embodiment, the body 712 is formed from a first materialthat is an electrical insulator, and the conduit 714 is formed from asecond material that is conductive. As such, the conduit 714 isconfigured to convey electricity through at least a portion of the body712. The pad 716 is conductive and is electrically coupled to theconduit 714. According to an exemplary embodiment, the materials of thebody 712, the conduit 714, and the pad 716 have been co-fired such thatcohesion therebetween fastens and hermetically seals the pad 716 and theconduit 714 with the body 712, forming a continuous interface betweenthe insulator body 712 and the pad 716, which is believed to beimportant for hermeticity. The continuous interface may also includecoatings, the base layer 718 (e.g., under-layer), or other intermediateelements. In some embodiments, the feedthrough 710 includes a second padcoupled to the conduit 714 on an opposite side of the conduit (see,e.g., pads 536, 538 as shown in FIG. 16), which may be of differentdimensions than the pad 716.

According to an exemplary embodiment, the pad 716 is sufficientlystructured (e.g., with regard to thickness, material type, surface area,surface flatness, layering, etc.) so as to support welding of a lead orwire (e.g., Nb lead; cobalt-chromium-nickel alloy (“Co—Cr—Ni alloy,”e.g., MP35N, 35N LT, Co—Cr—Ni alloy with nano-grain structure, ASTMstandard F562)) to a top surface of the pad 716 without significantlydamaging the hermetic seal between the pad 716 and the body 712. Manytypes of welding processes may be used including laser and parallel gapwelding techniques. Some representative external interconnect techniquesinclude laser welding, parallel gap welding, brazing, ultrasonicbonding, thermo-sonic bonding, soldering, diffusion bonding, andpressure or scraping contacts. Some representative external interconnector lead materials include niobium, platinum, titanium, tantalum,palladium, gold and oxides and alloys thereof (e.g., Ti₁₅Mo, PtIr,Co—Cr—Ni alloy, Grade 36 TiNb alloy). Although shown as generallyrectangular (e.g., square) in FIG. 19, in other contemplated embodimentsthe pad may be round or otherwise shaped. The shape may vary dependingupon design requirements, while having upper layers of the pad 716narrower than the base layer 718.

Referring now to FIG. 20, a pad 810 is coupled to an insulator 812 abovea conductive conduit 814. According to an exemplary embodiment, fromleft to right FIG. 20 shows configurations of the pad 810 as the pad 810is being constructed (e.g., printed), with the final form of the pad 810shown in the configuration 810C on the right. The configurations 810A,810B on the left and in the middle may be final forms of the pad 810according to other embodiments. The lower row of FIG. 20 includes actualmicrographs, provided by a scanning electron microscope, of threedifferent pads 810A′, 810B′, and 810C′ representative of the threeconfigurations 810A, 810B, and 810C shown in the upper row of FIG. 20.

According to an exemplary embodiment, the pad 810 includes a first layer816, and a second layer 818 overlaying at least a portion of the firstlayer 816. The insulator 812 is formed from a first material, theconduit 814 is formed from a second material, the first layer 816 of thepad 810 is from the second material, and, in some embodiments, thesecond layer 818 of the pad 810 is formed from a third material.According to an exemplary embodiment, the second material serves as anintermediary between the first and third materials to improve adhesion.In some embodiments, the first layer 816 of the pad 810 separates thesecond layer 818 of the pad 810 from the first material of the insulator812 such that the second layer 818 of the pad 810 is not in directcontact with the first material. In some such embodiments, the firstmaterial includes alumina, the second material includes platinum withalumina as an additive, and the third material includes primarilyplatinum.

In some embodiments, the pad 810, which may include layers 820, 822 inaddition to the first and second layers, has a thickness T of at least50 μm, such as at least about 75 μm or about 100 μm. In someembodiments, the pad 810 is less than 200 μm thick. Such thickness T isbelieved sufficient to allow for forming of a molten bead of material toweld a lead or wire to the pad 810, without melting the conduit 814 orseparating from the insulator 812. If the pad 810 is too thin, it hasbeen found that thermal stresses may cause the pad 810 or conduit 814 tocrack or delaminate from the insulator 812, damaging the connectivity ofthe associated feedthrough.

It is believed that interconnect pads for feedthroughs that are formedfrom platinum and are of a typical thickness on the order of 10 to 15 μmmay be too thin to receive leads using standard welding processes (e.g.,laser and parallel gap welding techniques), because it has been foundthat such pads deform or separate from the respective insulator, harmingthe hermetic seal of the feedthrough. Heat from the welding processesmay also pass through such pads to melt the underlying conduit, harmingthe hermetic seal of the feedthrough. On the other hand, pads on theorder of 10 to 15 μm (base pads) may be sufficiently thick forsoldering, brazing, or wire bonding processes, in contrast to welding.But soldering or brazing processes and associated materials may not bebiocompatible or biostable. With that said, in some contemplatedembodiments a pad having a thickness less than 50 μm, such as on theorder of 10 to 15 μm, or greater than 15 μm, may be used with certainpad materials or welding techniques. It should be noted that while thequantities and ranges provided herein may be useful in someconfigurations, in other configurations, such as those with othermaterials, geometries, used in other applications, etc., the quantitiesand ranges may be inapplicable, while the general teachings providedherein may still apply. For example, the dimensional thresholds of thepad may be based upon the particular details of the weld processevaluated, where if weld process configurations were changed; madelarger/smaller, lower/higher power, etc., the dimensional thresholdswould correspondingly change.

Still referring to FIG. 20, the pad 810C includes the third layer 820 onthe second layer 818, and a fourth layer 822 on the third layer 820. Insome such embodiments, the third and fourth layers 820, 822 are of thethird material, and are printed on the second layer 818 to increase thethickness of the pad 810 so that the pad 810 is configured to receive alead welded thereto. According to an exemplary embodiment, the fourthlayer 822 is the top layer of the pad 810, and has a top surface area ofmore than 10×10 mil (i.e., 1/100 inch by 1/100 inch) in magnitude (e.g.,may have circle, square, rectangle, or other shapes). In someembodiments, the top surface area is more than about 20×20 mil, such asabout 30×30 mil or about 40×40 mil. Such a surface area on the top ofthe pad 810 is believed to be sufficiently large to allow for forming ofa molten bead of material to weld a lead or wire to the pad, withoutmelting sides of the pad 810 or separating the pad from the insulator812, which would harm the hermetic seal. It is believed that the padsformed from platinum having surface areas that are less than about 30×30mil may be too small to receive leads in some standard weldingprocesses, because such pads have been found to melt and separate fromthe body, harming the hermetic seal of the feedthrough. However, incontemplated embodiments a pad having a surface area less than 30×30 milmay be used with certain pad materials or welding techniques. It shouldbe noted that volumes and ranges of volumes of pads, according tovarious embodiments, include the product of any pad areas and any padthicknesses disclosed herein, or the product of any pad lengths, widths,and thicknesses disclosed herein.

According to an exemplary embodiment, the surface of the top of the pad810 is sufficiently flat so as to facilitate welding of a lead or wireto the surface. In some such embodiments, the top of the pad has a rootmean square value of less than about 10 μm for flatness, such as lessthan about 7 μm for flatness, where the area measured for flatnesscorresponds to the center 50% of the top of the pad (e.g., centralcircle in circular pad, central rectangle in rectangular pad). In othercontemplated embodiments, pads are designed to project vertically,forming a posting for connection of a lead or other interconnect.Parallel gap welding or laser welding may be used to fasten a lead to aposted protrusion.

Various conductive pastes reformulated from platinum powders may be usedto form conductive features (e.g., conduit, pad) of feedthroughs in someembodiments. A first paste is formed from a first platinum powderconsisting essentially of platinum having an average particle sizedistribution d₅₀ (mass-median-diameter in log-normal distribution) inthe range of 3-10 μm (“Pt-1”). A second paste is formed from a secondplatinum powder consisting essentially of platinum having a coarseraverage particle size distribution d₅₀ in the range of 5-20 μm (“Pt-2”).A third paste is formed from a combination of about equal parts of thefirst and second platinum powders and about 2-10% by weight alumina(e.g., Al₂O₃), such as about 5% alumina. A fourth paste is formed fromthe first and second powders mixed together at a ratio of about 3:1(e.g., 70-80%), respectively.

Mixing of the first and second powders in the third and fourth pastes isintended to control the sintering shrinkage and/or shrinking profile ofthe resulting metallization. In one example, paste formed from a 7:3mixture of the first and second powders and about 5% alumina additiveresulted in 13% shrinkage in thermo-mechanical analysis (TMA). Inanother example with the same mixture of first and second powders andabout 7% alumina, the shrinkage was 12%. In another example, pasteformed from a mixture of about equal parts of the first and secondpowders and about 5% alumina, resulted in 15% shrinkage, while the samemixture with 7% alumina resulted in 13% shrinkage.

By way of examples provided for context, various combinations of thepastes and numbers of layers have been constructed to test thequalities, such as top pad thickness and flatness, of the resulting padsfollowing co-firing. In two such examples, a top layer of the secondpaste was printed atop a base layer of the third paste (e.g., “doubleprinting”) and co-fired, resulting in top pad thicknesses of 37 and 39μm (e.g., average of 10-20 sample measurements per pad), respectively,and with root mean square (RMS) average flatness values of 4.2 and 3.9μm, respectively (see generally pad 810A as shown in FIG. 20). Inanother example, a top layer of the first paste was printed atop a baselayer of the third paste, which resulted in a top pad thickness of 130μm and RMS average flatness value of 12.3 μm. In another two examples,two top layers of the second paste were printed atop a base layer of thethird paste (e.g., “triple printing”) and co-fired, resulting in top padthicknesses of 55 and 59 μm, respectively, and with RMS average flatnessvalues of 2.5 and 3.3 μm, respectively (see generally pad 810B as shownin FIG. 20). In yet another two examples, three top layers of the secondpaste were successively printed atop a base layer of the third paste(e.g., “quadruple printing”) resulting in top pad thicknesses of 81 μmand RMS average flatness values of 3.9 and 4.2 μm, respectively (seegenerally pad 810C as shown in FIG. 20). In another example, three toplayers of the first paste were successively printed atop a first layerof the third paste resulting in a pad thickness of 109 μm and an RMSaverage flatness value of 6.0. In still another example, three toplayers of the fourth paste were successively printed atop a first layerof the third paste resulting in a pad thickness of 104 μm and an RMSaverage flatness value of 5.1, which resulted in a pad coupled to theunderlying conductive conduit (of the third paste) and to the insulatorwithout cracking or delamination. The net thickness of the top layersand base layer was 136 μm.

Various pad configurations were constructed using pastes formed fromcombinations of Pt-1 platinum powder, Pt-2 platinum powder, and platinumpowder formed from equal parts of Pt-1 and Pt-2 (“Pt-3”). FIG. 20 showsexamples of such configurations. The following table, provided forcontext, summarizes screening of various pad structures:

TABLE 9 Structure Pt-2 Pt-2 Pt-2 Pt-2 Pt-1 Pt-2 Pt-2 Pt-2 Pt-2 Pt-2 Pt-3Pt-3 Pt-3 Pt-3 Pt-3 Pt-3 Pt-3 Thickness  37 μm  39 μm  55 μm  59 μm  81μm  130 μm Flatness 4.2 μm 3.9 μm 2.5 μm 3.3 μm 3.9 μm 12.3 μmwhere the “Structure” row shows the layers of platinum paste in verticalorder, the “Thickness” row shows the thickness of the top pad (above thePt-3 layer), and the “Flatness” row shows the root mean square averageflatness values. Platinum powder formed from three parts Pt-1 to onepart Pt-2 (“Pt-4”) was used and the pad structures were further refined,as summarized in the following table provided for context:

TABLE 10 Structure Pt-2 Pt-1 Pt-4 Pt-2 Pt-1 Pt-4 Pt-2 Pt-1 Pt-4 Pt-3Pt-3 Pt-3 Thickness  81 μm 109 μm 104 μm Flatness 4.3 μm  6.0 μm  5.1 μm

where the rows match those of TABLE 9. The structure formed fromquadruple printing of Pt-4 and Pt-3 layers showed no signs of crackingalong the pad edge and showed relative flatness. In at least oneembodiment, the top of the pad was formed by three stacked layers ofPt-4 with a base layer of the pad, and via (including intermediate coverpads) composed of Pt-3 with 5% alumina additive.

By way of examples provided for context, highly-accelerated immersiontesting for dye infiltration at 150° C., 3.5 atm, for 30 days followinga 1 hour, 500° C., vacuum pre-heating, was conducted on samplefeedthroughs formed from various combinations of the pastes. Despiteinitial measurements indicating hermeticity before testing, duringtesting it was surprising to find evidence of loss of hermeticity (e.g.,dye infiltration) in feedthroughs constructed with pads (e.g., top pad,main pad), cover pads (e.g., pads in between layers), and conduits(e.g., via) all formed from the first paste in alumina insulators, aswell as those all formed from the first paste plus a lesser amount ofalumina additive (e.g., about 2.5%). By contrast, no loss of hermeticitywas found in feedthroughs constructed with pads, cover pads, andconduits formed from the first paste plus a greater amount of aluminaadditive (e.g., about 5% and about 7.5%). Also, no loss of hermeticitywas found in feedthroughs constructed with pads and cover pads of thesecond paste and conduits (e.g., via) between the cover pads formed frompaste formed from the first powder and 5% alumina additive. It isbelieved that using only alumina as an additive, as opposed to furtherincluding SiO₂, MgO, and CaO, decreases initial defects between theconduit and insulator.

The pad, in some embodiments, may be sufficient to maintain hermeticityand long-term biostability regardless of the composition and interfaceof the conductive conduit. In other embodiments, the pad may conductelectricity to the conduit, but may not be designed to prevent ingressof bodily fluids. In some such embodiments, the conductive conduit maybe formulated and structured to provide a hermetic seal and long-termbiostability to the feedthrough. It should be noted that improvedreliability may be provided by pads and conductive conduits thattogether are redundantly hermetically-sealed and long-term biostable.

Referring once again to FIG. 21, a portion of the method 1010 ofmanufacturing a feedthrough 924 includes providing 1012 the sheet 938 orbody of the first material 916. In some embodiments, the sheet 938 has aconduit 918 of a second material 940 extending through a hole 936 in thefirst material 916. The first material 916 is an electrical insulatorand the second material 940 is conductive. The method 1010 furtherincludes printing 1018 a first layer (see, e.g., first layer 816 asshown in FIG. 20 and base layer 718 as shown in FIGS. 18-19) of a pad946 (e.g., interconnect, top pad) on the sheet 938. The first layeroverlays the conduit 918 and is electrically coupled to the conduit 918.In some embodiments, the first layer of the pad 946 is formed from thesecond material 940.

According to an exemplary embodiment, the method further includesprinting 1018 a second layer (see, e.g., second layer 818 as shown inFIG. 20) of the pad 946 on top of the first layer. The method furtherincludes co-firing the sheet 938, the conduit 918, and the pad 946 suchthat cohesion therebetween fastens and hermetically seals the pad 946and the conduit 918 with the sheet 938. Printing 1018 of multiple layers(see generally pad 810C as shown in FIG. 20) for the pad 946 allows forincreased thickness of the pad 946, as may facilitate welding of a leador wire to the pad while maintaining a hermetic seal between the pad946, the conduit 918, and the sheet 938. Control of the dimensions ofthe pad 946 by printing 1018 multiple overlapping layers of the pad 946allows for formation of a pad configured for use with an implantablemedical device, because the pad 946 may be formed from biocompatible andbiostable materials (e.g., platinum) arranged to be thick enough, wideenough, and flat enough for welding, while maintaining a hermetic sealwith the body of the feedthrough 924.

While teachings disclosed herein relate generally to implantable medicaldevices (see, e.g., devices 110, 210 as shown in FIGS. 1-2), thedisclosure is not intended to be limited to such devices. For example,some of the teachings disclosed herein relate to methods and structuresthat provide for a hermetic feedthrough, formed from a co-firingprocess. On a micro-scale, features that allow for a hermetic seal thatremains biostable over a long duration (e.g., years), also providestrong, reliable bond between the insulator and the conductivecomponents of the feedthrough. Such improved bond may be beneficial fornon-medical, non-implantable devices undergoing conditions requiringhigh reliability and/or long-term hermeticity for the components of afeedthrough, such as computers that experience large changes intemperature, operate in chemically aggressive environments, electricaldevices that experience relatively high vibratory loading (e.g.,aircraft electronics), high-value devices robustly constructed, andother devices.

In implantable medical device applications, it may be desirable toemploy implantable medical devices, including portions thereof (e.g.,feedthroughs), that are non-magnetic and are compatible with diagnostictools that utilize magnetic fields, such as magnetic resonance imaging(MRI) systems. In some embodiments, the platinum and alumina materials,compositions, pastes, etc. disclosed herein (e.g., via paste, insulatormaterial, pad material) are non-magnetic and are compatible with MRI andother magnetic diagnostic techniques.

The construction and arrangements of the feedthrough, as shown in thevarious exemplary embodiments, are illustrative only. Although only afew embodiments have been described in detail in this disclosure, manymodifications are possible (e.g., variations in sizes, dimensions,structures, shapes and proportions of the various elements, values ofparameters, mounting arrangements, use of materials, colors,orientations, etc.) without materially departing from the novelteachings and advantages of the subject matter described herein. Someelements shown as integrally formed may be constructed of multiple partsor elements, the position of elements may be reversed or otherwisevaried, and the nature or number of discrete elements or positions maybe altered or varied. The order or sequence of any process, logicalalgorithm, or method steps may be varied or re-sequenced according toalternative embodiments. Other substitutions, modifications, changes andomissions may also be made in the design, operating conditions andarrangement of the various exemplary embodiments without departing fromthe scope of the present invention.

What is claimed is:
 1. A hermetic feedthrough for an implantable medicaldevice, comprising: a sheet having a hole defined therethrough, whereinthe sheet comprises a first material that is an electrically insulativeceramic comprising alumina; a second material substantially filling thehole so as to form a conduit, the second material comprising platinumand an additive that comprises alumina, and wherein the second materialdoes not include SiO₂, MgO, or CaO; wherein the first and secondmaterials have a co-fired bond therebetween, the co-fired bondhermetically sealing the hole.
 2. The hermetic feedthrough according toclaim 1, further comprising a cover pad positioned on the conduit. 3.The hermetic feedthrough according to claim 2, wherein the cover padcomprises a plurality of layers.
 4. The hermetic feedthrough accordingto claim 3, wherein the second material is electrically conductive. 5.The hermetic feedthrough according to claim 4, wherein at least onelayer of the cover pad comprises an electrically conductive thirdmaterial having a different composition than the second material.
 6. Thehermetic feedthrough according to claim 5, wherein the second materialand the third material have a co-fired bond therebetween.
 7. Thehermetic feedthrough according to claim 3, wherein the cover pad has athickness of at least 50 micrometers.
 8. The hermetic feedthroughaccording to claim 2, wherein the cover pad extends at least partiallyover the sheet past the conduit.
 9. The hermetic feedthrough accordingto claim 2, wherein: the cover pad is mounted to an exterior surface ofthe sheet and is structured to receive a lead connected thereto, thecover pad being electrically conductive and coupled to the conduit; andthe sheet and the cover pad have a co-fired bond therebetween, theco-fired bond hermetically sealing the cover pad with the sheet, and thehermetic seal being bio stable such that immersion durability ismaintained after attachment of the lead to the cover pad.
 10. Thehermetic feedthrough according to claim 2, wherein the cover pad iscentered over the conduit.
 11. The hermetic feedthrough according toclaim 2, wherein the cover pad comprises an underlayer of the secondmaterial.
 12. The hermetic feedthrough according to claim 11, whereinthe cover pad further comprises an additional layer of a third materialoverlaying the underlayer.
 13. The hermetic feedthrough according toclaim 12, wherein the third layer consists essentially of platinum, andthe underlayer comprises alumina and platinum.
 14. The hermeticfeedthrough according to claim 12, wherein the second material and thethird material have a co-fired bond therebetween.
 15. The hermeticfeedthrough according to claim 1, wherein the second material consistsessentially of platinum and an alumina additive.
 16. The hermeticfeedthrough according to claim 1, wherein the second material comprisesa first platinum powder including particles of a first size range, and asecond platinum powder includes particles of a second size rangedifferent from the first size range, and wherein the alumina of thesecond material is between two to ten percent of the second material byweight.
 17. The hermetic feedthrough according to claim 16, wherein thefirst material comprises 70% by weight of alumina.
 18. The hermeticfeedthrough according to claim 1, wherein the hermetic feedthrough iscoupled to an encasement structure of the implantable medical device.19. The hermetic feedthrough according to claim 1, wherein the sheet isa first sheet and the hole is a first hole, and further comprising asecond sheet layered on the first sheet and having a second hole definedtherethrough, the second hole aligned with the first hole.
 20. Thehermetic feedthrough according to claim 19, wherein: the second sheetcomprises the first material, and the second material substantiallyfills the second hole.